Catalytic Partial Oxidation of Methane with Rhodium Catalysts for Hydrogen Production

Transcription

1 Catalytic Partial Oxidation of Methane with Rhodium Catalysts for Hydrogen Production L.D.Vella 1, S. Specchia 1, B. Lorenzut 2, T. Montini 2, P. Fornasiero 2, V. Specchia 1 1. Dipartimento di Scienze dei Materiali ed Ingegneria Chimica - Politecnico di Torino, Torino - ITALY 2. Dipartimento di Scienze Chimiche and INSTM - Università di Trieste, Trieste ITALY 1. Introduction Natural gas (NG) is an abundant source of energy, but large portions of the world s reserves of NG remain virtually untouched and need to be effectively used. Almost all options for NG exploitation involve its initial conversion to syngas (H 2 and CO) that can then be used in a wide range of applications. Actually the most important industrial route to syngas production is steam reforming of CH 4 ; this process is carried out in large-sized reactors with high energy demand [1], suffering mainly of high energy consumption, high investment costs and H 2 /CO ratios 3 which do not suit all important downstream processes [2]. Catalytic partial oxidation (CPO) of CH 4 offers a promising alternative; it is a mildly exothermic process (H = kj mol -1 ) with H 2 /CO ratio of about 2, resulting appropriate for subsequent methanol production or Fischer-Tropsch processes for synthetic liquid fuel. Moreover, the procurement of H 2 for fuel cells, in particular in their application for the propulsion of future vehicles, presents a further driving force for a more compact syngas production technology [1]. CPO of CH 4 over metal catalysts was intensively studied; nickel [3-5], cobalt [5-7] and noble metal based catalysts [8-10] showed high activities. It is widely accepted that with metal catalysts a first oxidation of methane to CO 2 and H 2 O in the first part of the catalytic bed until O 2 is exhausted, is followed in the final portion of the catalytic bed by reforming of the remaining CH 4 with CO 2 and H 2 O initially formed [8-10]. However, at extremely high temperatures and very short contact time (SCT), it is possible that syngas is formed directly [9-13]. In this case, syngas is obtained by flowing through a small catalytic bed volume the reactants for few milliseconds and producing H 2 and CO through direct partial oxidation reactions. This approach has been widely investigated both in its fundamental and technological topics [14]. Recent studies [15] have shown that fixed bed reactors in catalytic partial oxidation, offer the best performance for syngas production. The aim of the present work was the design of the optimal fixed bed structure made of Rh/-Al 2 O 3 particles obtained with two different preparation methods for syngas production from CH 4 and O 2 through a SCT-CPO reactor. 2. Experimental 2.1. Catalysts Preparation and Characterization The research was primarily focused on designing a catalyst thermally stable under extreme working conditions and resistant to deactivation by aging (coking and sintering). Two different structures based on Rh over Al 2 O 3 were developed: an egg-shell catalyst (hereafter named A) and egg-yolk catalyst (hereafter named B). The A catalyst carried Rh on the external support surface and the other, B, contained the same load of Rh embedded into the porous carrier. Both catalysts were loaded with 0.5% in weight of Rh. III-1, 1

2 The egg-shell Rh catalyst was prepared by Rh deposition over commercial -Al 2 O 3 spheres of 1 mm in diameter. Rh was deposited by incipient wetness impregnation technique, using 10% wt aqueous solution of Rh(NO 3 ) 3. The solution was added drop-wise at room temperature to the -Al 2 O 3 spheres which were stirred until no solution was left. The asprepared spheres were left at rest overnight and then calcined in air in an oven at 600 C for 6 h [16]. The egg-yolk Rh catalyst was synthesized starting from the preparation of stabilized Rh nanoparticles under Ar atmosphere at room temperature. HEAC16Br (a cationic surfactant) was used as protective agent for Rh metal particles. After the addition of Al(NO 3 ) 3 to the colloidal suspension of Rh nanoparticles, the resulting system was added drop-wise to NH 4 OH solution. The formed gel was aged and then filtered. Several washing cycles with NH 4 OH/NH 4 NO 3 buffer solution and distilled H 2 O were applied to remove the bromide ions from the surfactant. The obtained precipitate was suspended in 2-propanol under reflux, filtered, dried and calcined firstly at 500 C for 5 h and then for 5 h at 900 C [17]. Finally, the material was pressed, crushed and sieved to collect the fraction between 425 and 850 m. For both type of catalysts, 0.5% by weight of Rh was deposed to investigate the role of the two different structure. The morphology of as-prepared A and B catalysts was observed by FESEM (SEM FEI Quanta Inspect 200 LV apparatus) and TEM microscopy (Philips CM200 UT apparatus). The catalysts BET surface area was measured by means of N 2 adsorption with an automated gas sorption analyzer (Micromeritics ASAP 2010 M apparatus). The BET area of the two catalysts were high and very similar, between 152 and 157 m 2 g -1, in spite of the complete different preparation methods adopted. SEM micrographs enlightened an egg-shell distribution of the Rh for the A catalyst (Fig. 1), typical distribution due to the IWI method used for the catalyst preparation: the active component is present only in a thin external surface layer of the -Al 2 O 3 spheres but not in the internal pores. The thickness of the catalytic layer is approx 50 m. HRTEM analysis on B catalysts showed Rh nanoclusters of about 2 to 4 nm completely embedded in the Al 2 O 3 matrix, as expected with a typical egg-yolk configuration (Fig. 1). Fig. 1 SEM image of the A catalyst (left side: cross section of a sphere embedded in a resin matrix) and TEM image of B catalyst (right side). III-1, 2

3 2.2. Pilot Plant The experimental test can be divided into four main sections: (1) mixing and pre-heating; (2) reaction; (3) gas cooling; (4) gas analysis. In the first one, CH 4 and O 2 are mixed at room temperature and fed to the reactor where the mixture is pre-heated. The reactor was realized with two Inconel coaxial pipes (internal pipe: 15 mm i.d. and 2 mm wall thickness) with a resulting jacket which helps to improve the thermal insulation. The internal tube was covered with an oxidized layer of FeCrAlloy to avoid contacts between reactive gases and the nickel present in the Inconel alloy. The catalyst was arranged in a fixed bed of Rh/γ-Al 2 O 3 particles placed between two porous quartz disks. Upstream the catalytic bed there was a quartz particle bed (to complete the static mixing of O 2 and CH 4 ) followed by a SiC particle layer that promoted the pre-heating of the reagents mixture using the heat released from the reaction and provided a shield for the radiant energy from the reaction zone. After the reaction zone there was a layer of low thermal conductivity quartz particles to reduce the heat losses and allow the product stream to cool more slowly. The gas temperatures were monitored by two thermocouples located at the inlet and outlet of the catalytic bed, respectively. Once the products had left the reactor, a hygrometer determined the H 2 O concentration of the reaction gas mixture and afterwards part of the gas stream was sampled and analysed with a multiple gas analyzer (Uras, Caldos and Magnos modules from ABB) to measure H 2, CO, CO 2, CH 4 and O 2 concentrations. Then, the reacted gas stream was completely oxidized in a catalytic honeycomb burner, to avoid syngas release to the atmosphere. The CPO reaction was ignited by heating the reactor in a tubular oven up to 950 C and feeding a room temperature mixture of pure CH 4 and O 2 at a O 2 /CH 4 ratio equal to 0.5 (stoichiometric condition for partial oxidation). The reagents stream was heated by the hot particle segment upstream the catalytic bed, and once ignited the reactor remained thermally self-sustained by the heat released from the reaction. The start-up procedure took about 1 min and then the O 2 /CH 4 ratio was gradually increased to the operative desired value and maintained till to reach steady-state conditions. The chosen O 2 /CH 4 ratio to carry on the tests was 0.575, a value slightly above the stoichiometric one, which a previous research demonstrated to be the best one to maintain the reactor temperature in a proper range and obtain the best syngas yield [16]. The feed flow rate was adjusted accordingly to increase the WHSV from 130 to 450 Nl h -1 g cat Results and Discussion The so-prepared A and B catalysts were tested into the reactor in five different fixed bed structure configurations (1.5 g of catalyst were always present in the bed): (1) A catalyst fixed bed only; (2) B catalyst fixed bed only; (3) A/B catalysts mix 50%-50%, i.e. a fixed bed of a random A and B catalysts mixture; (4) A+B catalysts, 50%-50%, i.e. 1 st bed segment A catalyst, 2 nd bed segment B catalyst; (5) B+A catalysts, 50%-50%, i.e. 1 st bed segment B catalyst, 2 nd bed segment A catalyst. The obtained results from the catalytic activity tests of the different fixed bed configurations are reported in Figure 3 as CH 4 conversion, in Figure 4 as H 2 selectivity, in Figure 5 as CO selectivity, in Figure 6 as fixed bed inlet (T in ) and outlet (T out ) temperatures. The best value of CH 4 conversion was reached by B single catalyst, always over 90% in the whole range of WHSV. Instead the A single catalytic bed showed a lower performance and its conversion decreased always with the increase of the WHSV (Fig.2). The double catalyst beds had an intermediate behaviour between the two single catalyst beds. The same happened III-1, 3

4 to the H 2 selectivity curves where the best performance was reached by the B single catalyst showing values above 98% (Fig. 3). Fig. 2 CH 4 conversion vs. WHSV for different catalytic fixed beds. Fig. 3 H 2 selectivity vs. WHSV for different catalytic fixed beds. The CO selectivity curves had similar values, with an almost constant trend at medium-high WHSV except the B+A bed which started with lower values but reached to the same ones of A and A+B beds at high WHSV (Fig. 4). The inlet A catalyst bed temperature decreased significantly until 260 C with the increase of WHSV, while the outlet temperature increased until 1200 C; instead the inlet B catalyst bed had lower and more stable temperatures of A catalyst and always a T in higher than the corresponding T out (Fig. 5). III-1, 4

5 The different performance of the two type catalyst was probably due to different shape of the particles and Rh position on the γ-al 2 O 3 carrier; in the egg-shell configuration Rh is immediately accessible to reagents favouring short contact time reactions as direct partial oxidation of CH 4 while in the egg-yolk configuration the reagent must diffuse inside the pores of the catalyst thus promoting the total oxidation of CH 4 in the first part of the catalytic bed followed by the endothermic reforming reactions of the remaining CH 4 ; this was also confirmed by the higher T in and lower T out values recorded with catalytic beds where B catalyst was present in the fixed bed (see Fig. 5). Moreover, probably the irregular shape of particles of B catalyst increases the contact points between themselves by improving the heat transfer coefficient along the catalytic bed. Fig. 4 CO selectivity vs. WHSV for different catalytic fixed beds. Fig. 5 T in and T out temperatures vs. WHSV for the different catalytic fixed beds. III-1, 5

6 4. Conclusions Catalytic partial oxidation of NG for syngas production offers an excellent alternative to steam reforming. The present work investigated, in the WHSV range from 130 to 450 Nl h -1 g -1 cat, five different fixed bed structures using two different Rh-Al 2 O 3 -based catalyst both loaded with 0.5% by weight of Rh: a catalyst with Rh in egg-shell configuration and the other one with Rh in egg-yolk configuration. The best performances in term of CH 4 conversion (higher than 90%) and H 2 selectivity (higher than 98%) was obtained with the fixed bed realized only with the egg-yolk catalyst in the whole range of WHSV examined. Instead the fixed bed realized only with the egg-shell catalyst denoted the worst performance, decreasing with the increase of the WHSV. The fixed bed composed of various structures of the two catalysts denoted an intermediate behaviour between the two single catalyst beds. The different performance of the two catalyst type was probably due to the different shape of the particles and to the Rh position on the carrier itself. 5. References 1. Schwiedernoch, R., Tischer, S., Correa, C., Deutschmann, O.: Chemical Engineering Science, 58:633 (2003). 2. Zhu, J., Mujeebur Rahuman, M.S.M., van Ommen, J.G., Lefferts, L.: Applied Catalysis A General, 259:95 (2004). 3. Choudhary, V.R., Rane, V.H., Rajput, A.M.: Applied Catalysis A, 162:235 (1997). 4. Choudhary, V.R., Rane, V.H., Rajput, A.M.: Catalysis Letters, 22:289 (1993). 5. Choudhary, V.R., Rajput, A.M., Prabhakar, B.: Catalysis Letters, 15:363 (1992). 6. Choudhary, V.R., Sansare, S.D., Mamman, A.S.: Applied Catalysis A, 90:L1 (1992). 7. Choudhary, V.R., Rajput, A.M., Rane, V.H.: Catalysis Letters, 16:269 (1992). 8. Ashcroft, A.T., Vernon, P.D.F., Green, M.L.H.: Nature, 344:319 (1990). 9. Vermeriren, W.J.M., Blomsma, E., Jacobs, P.A.: Catalysis Today, 13:427 (1992). 10. Dissanayake, D., Rosynek, M.P., Kharas, K.C., Lunsford, J.H.: Journal of Catalysis, 132:117 (1991). 11. Hickman, D.A., Schmidt, L.D.: Science, 259:343 (1993). 12. Schmidt, L.D., Huff, M.: Catalysis Today, 21:443 (1994). 13. Hickman, D.A., Schmidt, L.D.: Catal. Lett., 17: 223 (1993). 14. Basini, L., Aasberg-Petersen, K., Guarinoni, A., Ostberg, M.: Catalysis Today, 64:9 (2001). 15. Hohn, K.L., Schmidt, L.D.: Applied Catalysis A General, 211:53 (2001). 16. Specchia, S., Negro, G., Saracco, G., Specchia, V.: Applied Catalysis B Environmental, 70:525 (2007). 17. Montini, T., Condò, A.M., Hickey, N., Lovey, F.C., De Rogatis, L., Fornasiero, P., Graziani, M.: Applied Catalysis B Environmental, 73:84 (2007). III-1, 6